1. Field of the Invention
The present invention relates to the synthesis of metal oxide nanopowders, and more specifically to a method of synthesizing metal oxide nanopowders by rapid microwave combustion.
2. Description of the Related Art
Nanoparticles, including nanopowders, nanoclusters and nanocrystals, are small particles that have at least one dimension less than 100 nm. Nanoparticle research is currently an intense area of scientific research due to a wide variety of potential applications in the biomedical, optical and electronic fields.
Nanoparticles are effectively a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nanoscale level this is often not the case. Size-dependent properties are observed, such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles, and superparamagnetism in magnetic materials.
The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. For bulk materials larger than one micrometer, the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material. The interesting and sometimes unexpected properties of nanoparticles are partly due to the aspects of the surface of the material dominating the properties in lieu of the bulk properties.
There are several methods for creating nanoparticles. Attrition and pyrolysis are common methods. In attrition, macro- or microscale particles are ground in a ball mill, a planetary ball mill, or other size-reducing mechanism. The method is simple and inexpensive, but can produce a broad particle size distribution, and the milling tool can possibly introduce contamination into the nanopowder.
In pyrolysis, a liquid or gas organic precursor is forced through an orifice at high pressure and burned. The resulting ash is air classified to recover the metal oxide nanoparticles. In the liquid phase method, nanoparticles are produced from a mixture of chemicals that react when heated to a certain temperature. The liquid phase method is effective, but there are additional costs resulting from the use of solvents and the production of large amounts of wastewater.
Gas phase synthesis methods are used extensively in industry to produce nanopowder metal oxide. These methods usually use supersaturated gases that are unstable relative to the formation of the solid material in nanoparticulate form. These methods can be classified by the phase of the precursor and the energy source used. The precursor could be in the solid, liquid or vapor phase. Conventional heating, solar energy, and laser or electromagnetic radiation energy can usually achieve conversion of the phase of the precursor from solid or liquid to gas phase.
A thermal plasma can also deliver the energy necessary to cause evaporation of small micrometer-size particles. The thermal plasma temperatures are in the order of 10,000K, so that the solid powder easily evaporates. Nanoparticles are formed upon cooling while exiting the plasma region. The main types of thermal plasma torches used to produce nanoparticles are dc plasma jet, dc arc plasma, and radio frequency (RF) induction plasmas. In the arc plasma reactors, the energy necessary for evaporation and reaction is provided by an electric arc that forms between the anode and the cathode. In RF induction plasma torches, energy coupling to the plasma is accomplished through the electromagnetic field generated by the induction coil. The plasma gas does not come in contact with electrodes, thus eliminating possible sources of contamination and allowing the operation of such plasma torches with a wide range of gases, including inert, reducing, oxidizing and other corrosive atmospheres. The working frequency is typically between 200 kHz and 40 MHz. Laboratory units run at power levels in the order of 30-50 kW, while the large-scale industrial units have been tested at power levels up to 1 MW. The RF plasma method has been used to synthesize different nanoparticle materials, for example, synthesis of various ceramic nanoparticles, such as oxides, carbours/carbides, and nitrides of Ti and Si.
None of the above methods, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, a method for synthesizing metal oxide solving the aforementioned problems is desired.